Polycrystalline diamond inserts (“PCD inserts”) often form at least a portion of a cutting structure of a subterranean drilling or boring tools; including drill bits (fixed cutter, roller cone and percussion bits,) reamers, and stabilizers. Such tools, as known in the art, may be used in exploration and production relative to the oil and gas industry. PCD inserts may also be utilized as wear or cutting pads on the gage of downhole tools in order to cut and/or maintain the hole diameter. Such a PCD insert may be known as a PCD gage insert. A variety of PCD gage inserts are known in the art.
Tensile stress zones are often developed due, at least in part, to the thermal expansion differences between polycrystalline diamond and a substrate to which the polycrystalline diamond becomes bonded to during a HPHT process. Accordingly, tensile stress may be present in nearly all PCD products. The manufacturing process of PCD inserts creates residual stresses that often include tensile stress zones in the polycrystalline diamond. Tensile stress zones or regions may also be developed in response to applied forces or moments (on either the polycrystalline diamond, the substrate, or both) in combination with residual stresses.
Diamond is a brittle material that will not sustain high tensile loading. Residual and applied load stresses combined can significantly affect the performance of a PCD insert (e.g., a PCD gage insert). A polycrystalline diamond PCD gage insert (otherwise known as a diamond enhanced insert or “DEI”) may be manufactured by various methods which are known in the art. For example, one process includes placing a substrate adjacent to a layer of diamond crystals in a refractory metal can. Further, a back can is then positioned over the substrate and sealed to form a can assembly. The can assembly is then placed into a cell made of an extrudable material such as pyrophyllite or talc. The cell is then subjected to conditions necessary for diamond-to-diamond bonding or sintering conditions in a high pressure/high temperature press.
Accordingly, tensile stresses developed within any portion of polycrystalline diamond, are believed to be detrimental to DEIs, gage elements, or wear elements (e.g., as used on subterranean drilling tools). Such tensile stresses are also believed to contribute to premature damage (e.g., spalling, chipping, or delamination) of the polycrystalline diamond. On the other hand, some residual stresses are believed to be beneficial. Particularly, compressive stress developed within the polycrystalline diamond of a PCD insert are believed to be beneficial and may improve the durability of the polycrystalline diamond during use. Moderate to relatively high compressive residual stresses within a polycrystalline diamond table or layer may inhibit fracture initiation and development.
Conventionally, residual stresses have been managed via the diamond/substrate design (e.g., an interface between the polycrystalline diamond and the substrate, size of the diamond and/or substrate, shape of the diamond and/or substrate, etc.). Other methods for affecting residual stresses, including, for example, transition layers between the diamond and carbide to provide a gradient of thermal expansion properties, are known in the art. Such residual stress management methods may create residual stresses that, to a limited extent, improve toughness of a PCD insert.
However, in addition to residual stress developed within a PCD, a mounting process for affixing a PCD insert to a drilling tool (e.g., brazing or press fitting the insert for attachment to the tool) may influence the stresses within the PCD insert. More particularly, press fitting or brazing will apply forces to a PCD insert that will influence and complicate the residual stress state. Generally PCD gage inserts are mechanically attached to a downhole tool by a press or interference fit. An interference fit induces compressive stresses on the enclosed material, which is typically a portion of the substrate of a PCD insert. The inference fit may create a bending moment on the exposed portion of the PCD insert. As discussed below, finite element analysis (FEA) predicts that a peripheral ring of tensile stress in the diamond table will develop due to residual stresses and the stresses developed by press fitting a conventional PCD insert, which is also described below, within a hole.
FIGS. 1, 2, and 3 show a perspective view, a schematic, side cross-sectional view, and a partial, enlarged, side cross-sectional view of a conventional DEI 10 comprising a substrate 12 and a diamond layer 20. More particularly, referring to FIGS. 1-3, a radius 16 is formed on a peripheral edge of the diamond layer 20, wherein a cross-sectional shape of the radius 16 is substantially a quarter circle (e.g., a circular arc formed by 90° central angle). Of course, one of ordinary skill in the art will understand that this radius feature may be annular and is generally formed upon a circumferential edge region of the diamond layer 20. In further detail, side surface 24 of diamond layer 20 as well as substantially planar surface 22 of diamond table 20 are both substantially tangent to the radius 16 (for a given cross-sectional plane) at respective intersection edges or lines. Such a configuration may be referred to as a “one-quarter radius.” Also, manufacturing processes for forming a one-quarter radius may often include a break out angle that causes the substantially planar surface 22 and the side surface 24 of the diamond layer 20 to not be exactly tangent to the curve forming the radius 16.
FIG. 4 shows a partial sectioned view of conventional DEI 10, wherein DEI is shaded according to data representing a stress field within the conventional DEI 10 shown in FIGS. 1-3. Particularly, FIG. 4 was generated by using finite element analysis to simulate the residual stresses developed during HPHT sintering of the diamond layer 20 and substrate 12 as well as stresses developed in response to press fitting the substrate within a hole formed in a steel material (e.g., an applied pressure or force about at least a portion of the periphery of the substrate). As shown in FIG. 4, a substantially continuous, circumferentially extending zone or region 31 of tensile stress is indicated proximate to the radius 16. As shown in FIG. 4, a tensile stress of about 5.746 104 psi. may be developed. Such a tensile stress zone may be detrimental if the DEI 10 is used a cutting or wear element on a subterranean drill bit, because typically at least a portion of the radius 16 may be forced against a subterranean formation and, therefore, may be subjected to relatively high additional localized applied stresses.
FIGS. 5 and 6 show a schematic side cross-sectional view and a partial enlarged side cross-sectional view of another conventional DEI 50 comprising a diamond layer 51 and a substrate 54, wherein a relatively small (e.g., 0.010 inch) chamfer 52 is formed on a peripheral edge of the diamond layer 52 (i.e., between planar surface 56 and side surface 58 of diamond layer 51) at a 45° angle θ with respect to planar surface 56 of diamond layer 51. As known in the art, an interface between diamond layer 51 and substrate 54 may be nonplanar. FIG. 7 shows a further conventional DEI 60 comprising a diamond layer 61 and a substrate 64, wherein a relatively large (e.g., 0.040 inches-0.070 inches) chamfer 62 is formed on a peripheral edge of diamond layer 61 (i.e., between planar surface 66 and side surface 68 of diamond layer 61). As shown in FIG. 7, chamfer 62 is formed at a 45° angle θ with respect to planar surface 66 of diamond later 61. FIG. 8 shows yet an additional conventional DEI 70 comprising a diamond layer 72 and a substrate 74, wherein the diamond layer 72 forms a substantially hemispherical surface 76. Generally, each of these conventional DEIs may exhibit undesirable tensile stresses within at least a portion of their respective polycrystalline diamond structure.
Thus, it would be advantageous to provide a superabrasive insert (e.g., a polycrystalline diamond insert) with a selected arcuate peripheral surface geometry. In addition, it would be beneficial to provide a superabrasive insert exhibiting a selected peripheral surface that produces, at least in part, an associated beneficial residual stress field. Of course, subterranean drill bits including at least one such polycrystalline diamond insert may also be beneficial.